Method for inhibiting corrosion of ferrous alloys



United States Patent 3,477,861 METHOD FOR INHIBITING CORROSION OF FERROUS ALLOYS Herman Wissenberg, Munster, Ind., assignor to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Filed Dec. 14, 1966, Ser. No. 601,539 Int. Cl. C09k 3/00; C23f 11/00; C09d /08 US. Cl. 106-14 3 Claims ABSTRACT OF THE DISCLOSURE The corrosive activity of bromine-containing compounds normally present in benzoic acid is inhibited by adding water to the benzoic acid in an amount suflicient to substantially suppress said corrosive activity.

This invention relates to a method of inhibiting the corrosion of normally highly corrosion resistant ferrous alloys.'The invention particularly concerns a method for inhibiting the corrosion of normally highly corrosion resistant ferrous alloys by contaminated benzoic acid when the contaminants are bromine-containing compounds.

Typically, in the production of benzoic acid by the bromine-catalyzed air-oxidation of toluene, small amounts of a myriad of bromine-containing compounds are formed. These bromine-containing compounds although in small amounts in the benzoic acid can be inorganic and/or organic in nature and may range in amounts from just above zero to several thousand parts per million (p.p.m.). The total amount of impurities including the bromine-containing compounds in the benzoic acid medium can be present in amounts of about 1% or less. Due to the presence of these bromine-containing compounds in the benzoic acid medium, although in such small amounts, a significant corrosion problem exists.

conventionally, in manufacturing and in other processes wherein it is necessary to retain benzoic acid having bromine-containing compounds, as well as in storage and transportation, the corrosion of highly corrosion resistant ferrous alloys remains a problem. For example, at one manufacturing concern, Whose operation necessitated the use of Type 316 Titanium stainless steel tanks for the storage of liquid benzoic acid, there was experienced an unusual amount of tank corrosion. Upon investigation, it was found that the small amounts of bromine-containing compounds caused the corrosion. Normally, benzoic acid per se is not corrosive in this particular ferrous alloy environment.

The known art exemplifies the inhibition of corrosion in many systems and in many Ways; however, no art has been found which anticipates nor even suggests arrest ing the corrosive attacks on highly corrosion resistant ferrous alloys by benzoic acid containing small amounts of impurities including bromine-containing compounds in the novel manner described and claimed herein.

It has been discovered that when liquid benzoic acid having small amounts of bromine-containing compounds therein is retained in highly corrosion resistant ferrous metal vessels, there is occurring a constant corrosion attack upon the ferrous metal container by contaminant bromine-containing compounds. However, when minor amounts of normally corrosive water are added to the benzoic acid medium retained by the ferrous metal vessel, corrosion inhibition surprisingly occurs and is maintained thereafter without substantial further addition or manipulation of the medium.

The benzoic acid having small amounts of brominecontaining compounds attacks corrosively any of the highly corrision resistant ferrous metals, such as, the molybdenum-containing austenitic stainless steels, including Type 317 and Type 317 ELC (extra low carbon),

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each containing molybdenum Within the range of 34% in accordance with the well known industry specifications. The bromine-contaminated benzoic acid also attacks Types 316, 316 ELC, 316 Ti (Titanium stabilized), and 316 Cb (Columbium stabilized). Industry specifications require that the Type 316 alloys contain 2-3% molybdenum. Ordinarily, benzoic acid having no bromine-containing contaminant compounds will not corrosively attack the Type 316 and Type 317 molybdenumcontaining austenitic stainless steels; however, some corrosion of the Type 304 alloys has been observed.

It is not fully understood how the mechanism through which the bromine-containing compound contaminants operate to cause the corrosive attack on corrosion resistant ferrous alloys, nor is it fully understood how the addition of minor amounts of a normally corrosive substance, that is, Water, to a substantially large body of contaminated benzoic acid suppresses the corrosiveness of the material. It is believed that when small amounts of bromide are present, the bromide may combine with the benzoic acid during manufacturing to form isomers thereof. It is thought that the ortho and meta isomers of bromo benzoic acid are less aggressive corrosively and that the para isomer, that is, para-bromo-benzoic acid, is the more aggressive of the isomers. It is also believed that other bromine-containing compounds form when bromide is present in small amounts in the benzoic acid medium. For example, alpha-bromo-toluene, among others, has been identified and appears to be an aggressive corrosion-producing agent.

The bromine-containing contaminant compounds in the benzoic acid medium can be present in very small amounts, such as a trace, to produce an observable corrosive effect upon molybdenum-bearing austenitic stainless steels. For example, at a level of parts per million bromine present as alpha-bromo-toluene, substantial corrosion of even the highly corrosion resistant ferrous alloys has been detected. Substantially increased corrosion activity has been observed at elevated levels of bromine contamination in the liquid benzoic acid medium, and measurements as high as 2200 parts per million bromine, present as an isomer of bromo benzoic acid have been made.

In accordance with this invention, to suppress the corrosiveness of benzoic acid having small amounts of bromine-containing compounds therein, minor amounts of normally corrosive water may be added to the comparatively large benzoic acid medium. The water can be ordinary well water having little or no halide content, or the water may be distilled. Distilled or deionized water is preferred. The Water may be added as a liquid, it may be injected as a steam, or added in its solid state as ice. For instance, water in any form may be added to the contaminated benzoic acid medium in amounts ranging from about 0.2% for small amounts of brominecontaining compounds. Good results are obtained when water is added in amounts ranging from about 0.5%- 5%, preferably, 0.6%4%, based upon the total weight of acid; however, satisfactory suppression of corrosion by alpha-bromo-toluene present in amounts ranging from 80-2200 parts per million bromine is accomplished by the addition of about 2.7% water. Satisfactory results demonstrating the desirable suppression of corrosion occurred when about 4% water was added when bromine was present as para-bromo-benzoic acid Within the range of about a trace to about 1000 parts per million. Generally, the eifectiveness of corrosion suppression by specific amounts of water added to a benzoic acid system having bromine-containing compounds depends upon the particular bromine compounds present, that is, although added water suppresses the corrosiveness of the brominecontaining compounds, it may not be necessary to add as much water for some of the corrosive contaminants as for other particular corrosive bromine-containing compounds.

In general, it is believed that stainless steels depend for their corrosion-resistance, on a tightly adherent film of oxide. If this oxide film were not adherent and itself resistant to attack, thermodynamic considerations suggest that the underlying stainless steel would dissolve rapidly in many environments including benzoic acid. It is Well known that certain halogen compounds, especially chlorides and bromides, are able to destroy part or all of the protective oxide film on a stainless steel, or, in lesser amounts, are able to prevent the repair of naturally occurring flaws or fissures in the film. However, it is believed that the addition of Water in this invention aids in the repair of the protective film. In other words, the water competes with the bromine-containing compounds in their effects on the stainless steel. The bromine-containing compounds tend to destroy the protective film while the water tends to mend the flaws in the film by reacting with the underlying metal and thereby forming a fresh supply of stable metal oxide.

Little is known about the effect of temperature upon the rate of corrosion activity in this benzoic acid system having small amounts of corrosion-producing brominecontaining compounds; however, it is observed that corrosion activity commences with a small increment of system temperature above the melting point of the benzoic acid medium, which is about 252 F. Generally, it appears that With small amounts of bromine-containing compoundspresent in the benzoic acid medium, temperature has little effect upon the corrosion activity rate, but subsequently greater temperature effect is observed when higher concentrations of corrosion-producing brominecontaining compounds exist within the benzoic acid medium.

The following examples are included to merely illustrate more specifically the details of this invention with no limitation of the scope of the novel subject matter described and claimed herein.

EXAMPLE I Specimens of Type 316 stainless steel were exposed in benzoic acid containing para-bromo-benzoic acid and distilled water was added to some of the specimens to obtain the following comparative data. The specimens were partly immersed in the liquid acid system having a temperature of about 326 F. for about 22 hours.

Bromine concentration derived from p-bromobenzoic acid (mg. Benzoic Br/kg. benzoic Water added Corrosion rate acid (g.) acid) (1111.) (mils/yr.)

The above data tends to show that with water addition to the corrosive acid system, Type 316 stainless steel vessels may be used for holding the benzoic acid.

EXAMPLE II The following data were collected in a procedure similar to that of Example I wherein specimens of Type 316 Bromine concentration derived from p-bromo-benzoic acid Water added (g. (mg. Br/kg. benzoic acid) water/ g. Corrosion rate benzoic acid (mils/yr.)

From the foregoing, it is apparent that through the addition of a normally corrosion-producing agent, namely, water, an unexpected and superior result is accomplished by the suppression of corrosion of ferrous alloys by a benzoic acid medium having a small amount of corrosion-producing bromine-containing compounds.

While in the foregoing specification this invention has been described in relation to preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to other embodir'nents and that many of the details set forth herein can be varied considerably without departing from the basic principles of the invention.

What is claimed is:

1. In the method of inhibiting the corrosive activity of bromine-containing compounds which are normally present in benzoic acid in amounts of about 1 percent or less by weight of the acid and which corrode ferrous metals, said corrosive activity commencing at temperatures above about 252 F., the step of adding water to the benzoic acid in an amount ranging from about 0.2 to about 5' weight percent of the acid.

2. The method defined in claim 1 wherein the brominecontaining compound is an isomer of bromo-benzoic acid.

3. The method defined in claim 1 wherein the bromine-containing compound is alpha-bromo'toluene.

References Cited UNITED STATES PATENTS 1,151,255 8/1915 Ferguson 25271.2 3,062,684 11/1962 Treseder et al. 10614 XR 3,375,077 3/1968 Bloch et al. 252-387 XR OTHER REFERENCES J. Grant: Hackhs Chemical Dictionary, 3rd ed., 1944, p. 608 relied on.

P. R. Frey: College Chemistry, Prentice-Hall, N.Y., 1952, p. 301 relied on.

JULIUS FROME, Primary Examiner L. HAYES, Assistant Examiner US. Cl. X.R. 252--3 87 

